and Clark (1985) and finally, the voluminous treatise on microstructures of Carter
(1990). Recent microstructural, mineralogical, and physical studies include the
works of Dauphin et al. (2003), of Checa et al. (2005), and of Esteban-Delgado
et al. (2008).
13.3.3 The Calcitic Prisms of Pinna nobilis
The data on the shell ontogeny of Pinna nobilis are not known, and all our
knowledge derives from developmental studies performed on bivalves with similar
shell texture, like Mytilus (Medakovic 2000). Following this general scheme, the
early shell appears during the trochophore stage, and is secreted by the transitory
shell gland, the group of cells resulting from the invagination of the ectoderm
(Kniprath 1981). After the production of an organic lamella – the future
periostracum – by the shell gland, the first mineralization is deposited between
the lamella and the shell gland itself. By analogy with other better-studied bivalves,
it is likely that the first minerals produced are amorphous (Mao Che et al. 2001;
Weiss et al. 2002). Then, the prismatic layer appears, by growing inward from the
organic lamella. At a later stage (veliger or post-metamorphosis?), the deposition of
the nacreous layer starts in the hinge area, while the prismatic layer continues its
lateral extension on the two valves.
Poorly investigated from an ontogenetic viewpoint, the prismatic layer has been
well studied in juvenile or adult specimens. As described by Boggild (1930), the
prisms, when seen in transverse section, “are very regular and, at the same time,
distinct, with an interprismatic substance. The orientation of the optic axis is unusually regular, parallel to the direction of the prisms.” Taylor and coworkers (1969)
distinguished two types of prisms, the simple ones and the composite ones. Those of
P. nobilis belong to the first category. These authors gave a more precise definition of
the prisms: “simple calcite prisms, arranged with their crystallographic c axes normal
to the layer surface. The prisms are larger than in any other family. . . and the
prismatic layer as a whole can be several millimeters thick.” In their scrupulous
attempt to classify molluscan shell microstructures, Carter and Clark (1985)
recognized several types of prismatic structures, defined as “mutually parallel, adjacent structural units (1
st order prisms) that do not strongly interdigitate along their
mutual boundaries. The first order prisms are generally longer than they are wide.”
Those of Pinna nobilis are defined as simple prismatic structures, “in which each 1
st
order prism shows a non-spherulitic prismatic and non-composite prismatic substructure. The boundaries of adjacent 1
st order prisms are well defined and generally noninterdigitating”. . .and further “they are commonly oriented with their length axes
perpendicular to the plane of the shell layer.” Furthermore, Carter and Clark (1985)
introduced a subcategory, of regular simple prismatic structures, in which “each
1
st order prism is columnar and has a more or less equidimensional (non-elongated)
polygonal cross sectional shape. The prism shapes and their diameter appear rather
uniform in section perpendicular to the prism length axes, although the prism
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
361
(1990). Recent microstructural, mineralogical, and physical studies include the
works of Dauphin et al. (2003), of Checa et al. (2005), and of Esteban-Delgado
et al. (2008).
13.3.3 The Calcitic Prisms of Pinna nobilis
The data on the shell ontogeny of Pinna nobilis are not known, and all our
knowledge derives from developmental studies performed on bivalves with similar
shell texture, like Mytilus (Medakovic 2000). Following this general scheme, the
early shell appears during the trochophore stage, and is secreted by the transitory
shell gland, the group of cells resulting from the invagination of the ectoderm
(Kniprath 1981). After the production of an organic lamella – the future
periostracum – by the shell gland, the first mineralization is deposited between
the lamella and the shell gland itself. By analogy with other better-studied bivalves,
it is likely that the first minerals produced are amorphous (Mao Che et al. 2001;
Weiss et al. 2002). Then, the prismatic layer appears, by growing inward from the
organic lamella. At a later stage (veliger or post-metamorphosis?), the deposition of
the nacreous layer starts in the hinge area, while the prismatic layer continues its
lateral extension on the two valves.
Poorly investigated from an ontogenetic viewpoint, the prismatic layer has been
well studied in juvenile or adult specimens. As described by Boggild (1930), the
prisms, when seen in transverse section, “are very regular and, at the same time,
distinct, with an interprismatic substance. The orientation of the optic axis is unusually regular, parallel to the direction of the prisms.” Taylor and coworkers (1969)
distinguished two types of prisms, the simple ones and the composite ones. Those of
P. nobilis belong to the first category. These authors gave a more precise definition of
the prisms: “simple calcite prisms, arranged with their crystallographic c axes normal
to the layer surface. The prisms are larger than in any other family. . . and the
prismatic layer as a whole can be several millimeters thick.” In their scrupulous
attempt to classify molluscan shell microstructures, Carter and Clark (1985)
recognized several types of prismatic structures, defined as “mutually parallel, adjacent structural units (1
st order prisms) that do not strongly interdigitate along their
mutual boundaries. The first order prisms are generally longer than they are wide.”
Those of Pinna nobilis are defined as simple prismatic structures, “in which each 1
st
order prism shows a non-spherulitic prismatic and non-composite prismatic substructure. The boundaries of adjacent 1
st order prisms are well defined and generally noninterdigitating”. . .and further “they are commonly oriented with their length axes
perpendicular to the plane of the shell layer.” Furthermore, Carter and Clark (1985)
introduced a subcategory, of regular simple prismatic structures, in which “each
1
st order prism is columnar and has a more or less equidimensional (non-elongated)
polygonal cross sectional shape. The prism shapes and their diameter appear rather
uniform in section perpendicular to the prism length axes, although the prism
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
361